Telephone is a VoIP program which allows you to make phone calls over the internet. It can be used to call regular phones via any appropriate SIP provider. If your office or home phone works via SIP, you can use that phone number on your Mac anywhere you have decent internet connection.

I love this application! Using VoIP services on the mac for a very long time now, I have to say this is the first app to reall work as expected!
The app is responsive, has a small footprint and totally rocks when it comes to integrating with Apple's AddressBook application. You can call directly from the AddressBook or just start typing the first letters of the person's name you want to call.

GREAT app! Easy to configure (once you figure out which info provided by your ISP goes into which field in the program) and an extremely simple dialling interface, linked in with the Apple Address Book. Also the app remains very responsive at all times. Much better than the dowdy 90s style X-Lite software I was using before. I am delighted with this, especially considering it's free!